Linear beam microwave tube having pole caps providing a tapered magnetic field along the beam axis



June 4, 1968 R. H. OHTOMO 3,387,167

LINEAR BEAM MICROWAVE TUBE HAVING POLE CAPS PROVIDING A TAPERED MAGNETIC FIELD ALONG THE BEAM AXIS Filed Nov. 6, 1964 FIG.|

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37 1 41/ A A 35 r L mfi D l LINEAR hf BTAPER H C {M Q INVENTOR. RICHARD H. OHTQMO AXIAL LE'NGTH 44 z' ATTORNEY United States Patent M 3,387,167 LHNEAR BEAM MKCROWAVE TUFBE HAVING POLE CAPS PRGVTDENG A TAPERED MAG- NETIC FKELD ALONG THE BEAM AXES Richard H. (inferno, Sunnyvale, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of Caiifornia Fiicd Nov. 6, 1964, Ser. No. 409,521 11 Claims. (Cl. 315-) ABSTRACT OF THE DISCLGSURE A permanent magnet focusing structure for linear beam traveling wave tubes. Permanent magnets disposed at each end of the slow wave circuit are equipped with I specially designed pole caps which cause the magnetic field to be tapered. The tapered magnetic field causes a substantial portion of the beam electrons to strike the slow wave circuit thereby enhancing electronic cfiiciency.

. to be found in US. patent application Ser. No. 330,728 by William L. Beaver, filed Dec. 16, 1963, now abandoned, and US. patent application Ser. No. 330,029 by Owen B. Pallakoff, filed Dec. 12, 1963, now Patent No. 3,283,- 200, both of which are assigned to the same assignee as the present invention. The present invention is an improved version of the permanent magnet focusing and shielding techniques 'as set forth in these aforementioned applications.

The present invention furthermore provides a linear beam traveling wave tube, particularly of the backward wave oscillator type, with enhanced electronic efficiency characteristics of operation as well as reduced overall permanent magnet focusing package weight requirements. The novel teachings of the present invention include the shielded focusing advantages of the aforementioned applications in conjunction with permanent magnet focusing. packages, while simultaneously providing enhanced electronic efiiciency characteristics through the utilization of a tapered magnetic focusing field along the electron beam axis.

The electronic efiiciency of a backward Wave oscillator can be improved by utilizing electron beam interception along the axial extent of the slow wave circuit thereby reducing the spacing between the exterior periphery of, for example, a hollow electron beam and the interior periphery of the surounding slow wave circuit. In order to provide such increased electronic efliciency, conventional beam space charge forces are permitted to cause beam expansion and impingement of beam electrons on the surrounding slow wave circuit along the axial extent thereof. The proper amount of beam interception and control of beam interception along the axial extent of the slow wave circuit is provided by the employment of a tapered magnetic focusing field along the axial extent of the slow wave circuit, which tapered field is advantageously provided by novel pole cap design techniques as taught by the present invention.

In order to reduce the focusing package weight requirements, the present invention utilizes high energy product permanent magnets in conjunction with tapered magnet designs, in order to maximize for a given design,

3,387,167 Patented June 4, 1968 magnet efliciency while simultaneously minimizing extraneous leakage fields. The present invention furthermore improves overall permanent magnet focusing efiiciency by reducing the gap field loss within the low reluctance path shield structure.

It is therefore the object of the present invention to provide a high frequency electron discharge device with improved permanent magnet focusing and shielding means.

A feature of the present invention is the provision of a novel shielded permanent magnet focusing package for linear beam electron discharge devices.

Another feature of the present invention is the provision of a high frequency electron discharge device of the linear beam traveling wave type which incorporates novel pole cap and magnet design characteristics.

Another feature of the present invention is the provision of a linear beam traveling wave tube having inlproved electronic efficiency characteristics.

These and other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional view, partly in elevation, of a high frequency linear beam electron discharge device of the traveling wave type disposed within a shielded permanent magnet focusing package;

FIG. 2 is a schematic view of the helix slow wave circuit and pole caps utilized in the apparatus depicted in FIG. 1 and accompanying illustrative graphical portrayal of focusing field along the axial extent of the slow wave circuit between the respective pole caps; and

FIG. 3 is an illustrative graphical portrayal of 'a typical hysteresis loop for high energy product permanent magnets advantageously utilized in the present invention.

Referring now to FIG. 1, there is shown a representative linear beam electron discharge device '6, of the traveling wave type, specifically a backward wave oscillator (BWO). Since the particular structural details of the backward wave oscillator 6 depicted in FIG. 1 do not form part of the present invention, the particular details thereof will not be gone into in detail. Suffice it to say that the electron beam, preferably hollow, is generated at the upstream portion of the oscillator 6 by means of an electron gun portion 7, said beam traversing the slow wave circuit; preferably a helix 8 as shown in FIG. 2, to the downstream portion terminated by exhaust tubulation 9. Electromagnetic wave energy is extracted from the oscillator through output coupling assembly 10, preferably of a conventional coaxial or waveguide coupler type, which includes a vacuum sealed wave permeable output window. For example of 'a typical prior art linear beam traveling wave tube of the backward wave oscillator type which is represented by tube 6, see US. Patent 2,991,391 by W. L. Beaver which is assigned to the same 'assignee as the present invention. A theoretical analysis of backward wave oscillators utilizing permanent magnet focusing is to be found in an article entitled Backward Wave Oscillators by H. R. Johnson, Proceedings of the I.R.E., vol. 43, No. 6, June 1955, pp. 684-697.

Suffice it to say for the purposes of the present invention as regards the patentable features thereof that a focusing and beam confining method is and must be employed in electron beam tubes in order to restrict and direct the electron beam through the interaction portion (slow wave circuit portion) of the device. The focusing package utilized in the present invention is generally indicated by 11, and includes a pair of low reluctance, high permeability 1,), preferably greater than 5000, shields or flux guides 12 and 13, made of materials such as, for example, soft iron or cold rolled steel and preferably of what is known as Armco magnetic ingot iron or hipernik. The fiux guides 12 and 13 are cup shaped as seen and joined at a portion 14 and securely fastened together by any suitable means, such as, for example, by internal rods and nuts as utilized in US. Patent No. 2,991,391 or by utilizing flanges at the mating edges and bolting the flanges together. In order to enhance the Permanent magnet efficiency requirements for the focusing package 11, while attempting to minimize the volumetric requirements, the mating portion 14 between shields 12 and 13 is provided with an increased thickness along the entire peripheral extent of the mating edges of portion 14. It

has been found that regardless of how tight a physical engagement is made between the mating edges of the two shields or flux guides 12 and 13 that a small air gap remains and causes considerable reduction in useful gap flux due to the increased reluctance thereof. Therefore, the beefed up sections 15, 16 of the shield walls have been found extremely useful in reducing the magnetic reluctance across the air gap between the mating edges. Therefore, the overall efiiciency and weight requirements of the focusing package are reduced accordingly.

Regardless of the fact that two approximately symmetrical cup shaped shields 12 and 13 have been depicted in the embodiment of FIG. 1 it is to be noted that any pair of shields, such as, for example, a can and cover embodiment dimensioned, for example, such as to be defined by dotted lines 46, could be employed in lieu of the symmetrical cup shaped shields 12 and 13, without departing from the scope of this invention. Obviously, the beefed up portions, portions having increased cross-sectional areas such as in the embodiment of FIG. 1, namely, and 16, would then be incorporated around the resulting air gap or mating edge between the can and cover portions in order to provide reduced loss.

A pair of axially polarized permanent magnets 18 and 19 are provided at the downstream and upstream end portions of the traveling wave tube 6, respectively. The permanent magnets 18 and 19 are annular cylinders, as shown, and are provided with a tapered external peripheral surface 20 between the respective poles, as shown. Any suitable bonding cement may be utilized to permanently affix the magnets 18 and 19 to the surrounding flux guide or shields 12 and 13, respectively, at the outer pole faces 21, 22 respectively. The inner axially oriented pole faces 23, 24 of each permanent magnet 18, 19 respectively, are provided with pole caps 25, 26 respectively, which are permanently affixed thereto by a suitable bonding cement. A pair of dielectric gaskets 29, 30,

'such as, for example, of Teflon, are disposed within stepped portions of the pole caps 25, 26 and provide a snug and stabilized fit between the exterior peripheral surface of the traveling wave tube and the focusing package.

In order to reduce the overall volume requirements and simplify the structural construction of the composite linear beam traveling wave tube and focusing package a high energy product permanent magnet material, for example, Alnico V7, manufactured by the Crucible Steel Company of America, is utilized as the permanent magnet materials of permanent magnets 18 and 19. It has been determined that, if permanent magnets 18 and 19 are provided with a cross-sectional area differential between the internal 23, 24 and external 21, 22 (inner and outer) pole faces with the cross-sectional area defined by the exterior peripheral surface of the magnet at the inner pole face reduced in comparison with the cross-sectional area defined by the exterior peripheral surface of the magnet at the outer pole face, increased operating efiiciency for the permanent magnet focusing package 11 results. A re sultant advantage is a reduction in overall volumetric requirements for a given permanent magnet material having a high energy product such as Alnico V-7 or Alnico V which have maximum energy products of around 7,000,- 000 gauss-oersteds.

Furthermore, by reducing the exterior diameter of the permanent magnets at the inner pole face or increasing the transverse distance X between the interior peripheral surface of the surrounding flux shields 12, 13 and the external periphery of the permanent magnets and pole caps at the inner pole faces 23, 24 leakage fields are reduced with a consequent increase in overall focusing package efiiciency and reduction in useless H fields. The preferred design requirements for the present invention, based on the assumption that high energy product magnet materials such as Alnico V and V7 permanent magnet materials (commercially available from Crucible Steel Company of America and Indiana General Corporation) are utilized to provide predetermined gap field of so many oersteds, and utilizing high permeability, preferably greater than 5,000, low reluctance materials such as soft iron, for example, Armco magnetic ingot iron for the shields 12 and 13, are as follows. A permanent magnet and surrounding fiux guide or shield, such as depicted in FIG. 1 should have the following relative dimensional parameters in order to enhance magnet operating efficiency while substantially reducing leakage fields and overall volumetric requirements. With reference to FIG. 1 and the physical dimensions delineated by A1, A2 and A3:

a a A, A

FIG. 3 discloses a typical hysteresis loop for a high energy product material such as Alnico V7.

Referring to FIGS. 1 and 3 the following definitions apply:

A =outer diameter of magnet at outer pole face A =inner diameter of shield at outer pole face A =outer diameter of magnet at inner pole face B =residual induction H =coercive force H :magnetizing force B=magnetic induction M =operating point of magnet In general for a given length magnet as the ratio I? the operating point M approaches H On the other hand,

'the curve at point M as shown in FIG. 3. This would result in the most efiicient magnet design. It can be shown for high energy product permanent magnet materials such as Alnico V and V7 utilized in conjunction with a surrounding fiux guide such as 12, 13 that enhanced gap H fields together with reduced leakage H fields and reduced permanent magnet volumetric requirements are realized when the aforementioned ratios are adhered to.

Directing your attention to FIG. 2, there is depicted a schematic view of the pole caps or pieces 25, 26 preferably of such high permeability low reluctance materials as soft iron together with a longitudinal cross-sectional view of the slow wave circuit 8 of the conventional helix type. A linear electron hollow beam 35, coaxial with central axis Z is shown traversing the axial extent of the slow wave circuit between the pole caps. As can be seen by the diverging dotted lines which are representative of the electrons present within the beam, electrons are constantly impacting or impinging on the slow wave circuit 8 between the upstream and downstream end portions thereof. In order to maximize the electronic efliciency in a backward wave oscillator, the present invention teaches preferably 50 to 100 percent beam impingement along the axial extent of the helix between the electron gun and the downstream end portion thereof. The aforementioned range of beam impingement percentages on the slow wave circuit has been found to result in enhanced electronic efficiency, wherein electronic efficiency can be defined as the ratio of the RF. power out over the DC. power input requirements for a backward wave oscillator.

In order to provide such beam impingement along the axial extent of the slow wave circuit 8, a tapered magnetic field, as shown by characteristic B in FIG. 2, is utilized. Characteristics B, C and D in FIG. 2 are illustrative representations of magnetic field intensity H along the axial extent or electron beam central axis Z between the pole caps. Characteristic C is representative of the magnetic field intensity along the central axis Z in the absence of any: geometric deformations of the pole caps with a pair of matched permanent magnets. In other words, assuming identical magnets and identical pole caps, characteristic C would result in a depression at the median plane represented by M which is undesirable.

Now, assume that the upstream or gun end pole cap is geometrically deformed such that it is represented by the pole cap 26 which has an axial protrusion or extension 37. The 'resultant distortion of the magnetic field intensity characteristic C induced by such a physical extension as 37 would result in a magnetic field characteristic along the axis Z such as D. Once again, although a tapered magnetic field has been achieved, there is an H-field depression between the downstream and upstream end portions which is undesirable. In order to remove the aforementioned H- field depression while further reducing the magnetic field intensity along the Z axis at the downstream end portion of the interaction circuit, the downstream end pole cap is provided with an axial extension 38 as shown in FIG. 2. The resulting H-field intensity characteristic between the pole caps along the Z axis is now represented by characteristic B which is shown to have an approximately linear taper along the Z axis. The degree of slope of this linear taper is, of course, a matter of design and can be varied according to the amount of beam interception desired. It has been determined that the ratio between the internal peripheral surface of the upstream pole cap extension 37 delineated by A the internal diameter of the pole piece extension 38 on the pole cap at the downstream pole delineated by A cap 25 and the axial distance between the extensions delineated by L should fall within the following limits:

The utilization of the concept of controlling the slope of the magnetic field intensity by variation in the trans- 'verse spacing of the internal diameters of the inwardly directed portions or extensions of the pole caps from the central axis permits simplicity of design and allows slope control without resorting to complex saturated pole cap techniques which involve highly critical control of the physical and electrical parameters of the focusing package. The aforementioned dimensions A A and L are more specifically defined as follows:

A ==diameter of axial bore through downstream pole cap 25 at the upstream portion of the extension 38. A =diameter of axial bore through upstream pole cap 26 at the downstream portion of the extension 37. L=,axial distance between pole caps It is to be noted that the extensions 37, 38 of the pole caps have an external peripheral configuration preferably similar to that of the external configuration of the permanent magnets themselves. Quite obviously, if rectangular or noncircular geometries are employed for the permanent magnets, similar geometries will be employed for the pole caps. The tapered magnetic field, delineated by characteristic B, between the upstream and downstream end portions of the backward wave oscillator 6 has been found to provide the necessary degree of beam impingement along the axial extent of the slow wave circuit in order to enhance the electronic efficiency of the oscillator. As power requirements are increased for backward wave oscillators, the degree of beam impingement will vary accordingly as determined by the acceptable or tolerable amount of current interception permissible for any given slow wave circuit. It has been found that beam interception along the axial extent of the slow wave circuit between the downstream and upstream end portions thereof, provides maximum electronic efficiency and that the electronic efficiency deviates accordingly as the amount of beam impingement is reduced.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A linear beam microwave tube apparatus having a linear central beam axis with upstream and downstream end portions including, means for providing a tapered magnetic field intensity along said beam axis, said means comprising a pair of magnets disposed at said downstream and upstream end portions and a pair of pole caps disposed at said downstream and upstream end portions, each of said pole caps having a central bore therein defining laxially directed extensions, the diameter of the axial 'bore through the downstream pole cap at the upstream portion of the extension being greater than the diameter of the axial bore through the upstream pole cap at the down stream portion of the extension.

2. The tube apparatus as defined in claim 1 wherein the ratio of said bores and the axial distance between said pole caps fall within the following limits:

A =diameter of axial bore through downsteam pole cap at the upstream portion of the extension.

A =diameter of axial bore through upstream pole cap at the downstream portion of the extension.

3. The tube apparatus as defined in claim 1 wherein said magnets :are fixedly secured to said pole caps and axially polarized and each of said magnets is provided with a tapered section along the axial extent thereof.

4. The tube apparatus defined in claim 1 wherein said tube is a backward wave oscillator including, a pair of high permeability magnetic flux guides surrounding said backward wave oscillator and said pair of magnets and pair of pole caps, said flux guides being joined together at mating edges, the thickness of each of said flux guides in the vicinity of said mating edge being greater than the thickness of each of said flux guides in portions removed from said mating edge.

5. A linear beam traveling Wave tube disposed along and defining an elongated central beam axis along which an electron beam travels between the upstream and downstream portions of said tube, a slow wave circuit disposed along said central beam axis between said upstream and downstream portions of said tube, means for controlling said electron beam as it traverses said slow wave circuit between said upstream and downstream portions of said tube, said means for controlling said electron beam being a magnetic field, said magnetic field extending along said central beam axis between said upstream and downstream portions of said tube, said magnetic field having a greater intensity along said central axis at the upstream portion than at the downstream portion of said tube, said magnetic field being provided by a pair of axially polarized permanent magnets, one of said pair of axial-1y polarized permanent magnets disposed at the upstream portion of said tube and the other of said pair of axially polarized permanent magnets disposed at the downstream portion of said tube, each of said pair of axially polarized permanent magnets being provided with a pole cap, each of said pole caps having axial extensions, said axial extensions having axially directed bores therein, the diameter of the axial bore through the downstream pole cap at the upstream portion of the extension being greater than the diameter of the axial bore through the upstream pole cap at the downstream portion of the extension.

6. The tube defined in claim 5 wherein a high permeability low reluctance flux guide surrounds said pair of permanent magnets and said tube, said flux guide comprising a pair of cup shaped members joined together at mating edges, the thickness of each of said cup shaped member being greater in the vicinity of said mating edges than in portions removed from said mating edges.

7. The tube defined in claim 5 wherein each of said permanent magnets in provided with a tapered section along the axial extent thereof.

8. An apparatus comprising a linear beam backward wave oscillator having a longitudinal axis along which an electron beam travels; a pair of axially polarized permanent magnets, one of said permenent magnets being positioned at one end of said backward wave oscillator thereby defining inner and outer axially oriented poles, the other of said permanent magnets being positioned at the other end of said backward wave oscillator thereby defining inner and outer axially oriented poles, the axis of polarization of said pair of permanent magnets being aligned with the longitudinal axis of said backward wave oscillator, said pair of permanent magnets having a magnetizing force capable of providing a focusing magnetic field along the longitudinal axis of said backward wave oscillator, and means forming a flux guide and shield substantially surrounding said linear beam backward wave oscillator and said pair of permanent magnets, said flux guide and shield being made of a low reluctance, high permeability material, said flux guide and shield being physically related to said pair of permanent magnets and said backward wave oscillator such that said flux guide and shield provide a low reluctance magnetic circuit between the outer axially oriented poles of said pair of permanent magnets and such that said flux guide and shield provide a magnetic shield for said backward wave oscillator and said permanent magnets, said pair of permanent magnets, each having a pole cap afiixed to the inner pole face thereof, said pair of permanent magnets in conjunction with said pair of pole caps being adapted and arranged to provide a tapered magnetic field intensity between the upstream and downstream portions of said linear beam backward wave oscillator, each of said pol caps having axial extensions, said axial extension-s having axially directed bores therein, the diameter of the axialbore through the downstream pole cap at the upstream portion of the extension being greater than the diameterofthe axial bore through the upstream pole cap at the downstream portion of the extension. 1;

9. The apparatus defined in claim 8 wherein eachof said permanent magnets is provided with a transverse tapered section between the inner and outer poles thereof. 10. The apparatus defined in claim 8 wherein the ratio of said bores and the axial distance between said pole caps falls within the following limits:

' where:

A =diameter of axial bore through downstream pole cap at the upstream portion of the extension. A =diameter of axial bore through upstream pole cap at the downstream portion of the extension. L=axial distance between pole caps. 11. The apparatus defined in claim 10 wherein the following dimensional relationships exist:

References Cited UNITED STATES PATENTS 2,730,648 1/ 1956 Lerbs 3 15-35 2,869,018 1/1959 Brewer et a1. 315-35 2,941,111 6/1960 Veith et a1 315-3.5 2,829,299 4/1958 Beck 3153.5 3,299,308 1/1967 Hanks 313-84 X HERMAN KARL SAALBACH, Primary Examiner.

PAUL L. GENSLER, Examiner. 

